It is sometimes desirable to configure a system to receive all of the electromagnetic signals within the receiver's capabilities as limited by its sensitivity and bandwidth. Signals of interest are usually incident from widely diverse directions. Therefore, prior systems have utilized antennas having a wide azimuth beam width such as omnidirectional antennas as the system's receptor.
A severe limitation of this approach is that it does not permit directional resolution of multiple signals. Such resolution is usually desirable to prevent garbling of signals that cannot otherwise be resolved in frequency or time-of-occurrence. Directional resolution is also desirable in cases where the direction of incidence of the signals is to be estimated.
To overcome these disadvantages, alternative prior art systems have been configured using narrow-beam antennas. In one case, multiple antennas, each producing a narrow beam, are arranged in a circular pattern so that their beams are contiguous and point radially outward. In another case, a single cylindrical array antenna is configured to form multiple beams which are contiguous and point radially outward. In both cases, each beam port of the antenna(s) is connected to a separate receiver, thus the system can exhibit the advantages of both good directional resolution and complete, simultaneous directional coverage. However, the disadvantage in this case is the high cost of the multiple receivers.
Another class of prior art systems attempts to achieve omnidirectional coverage with a single narrow beam by scanning that beam as a function of time. In these systems, a narrow beam is scanned over all azimuths by mechanical rotation of a fixed-beam antenna, or by electronic scan of a cylindrical array antenna. The disadvantage in this case is that the beam cannot look everywhere at once. This is especially a problem for multiple signals from diverse directions if they are nonrepetitive in character or have rapidly changing wave forms (high information rate or short-pulse signals). These high information rate signals may not be sampled at sufficient rate by the scanning beam to prevent information loss.
More recently, techniques have been disclosed which address the problems associated with directional resolution of multiple signals. A recent disclosures. U.S. Ser. No. 719,460, provided a cylindrical array antenna system capable of scanning a narrow beam through its complete coverage sector at a rate at least twice as fast as the maximum information rate of the signals it receives so that no information is lost. This allows the system to scan within the time period of the shortest pulse which it is expected to receive and thereby have a high probability of intercepting and receiving that signal. This system provided angular resolution of multiple signals and the capabilities of determining their direction of arrival commensurate with the narrow beam widths of a full N element cylindrical array. The system provided the same sensitivity and angular resolution regardless of the direction of signal incidence. These improvements were the result of using heterodyne techniques to achieve very rapid scanning of a single beam throughout the antenna's entire sector of coverage.
This technique, however, does result in a sensitivity loss due to sampling. This loss occurs because the scanning beam is only directed at the angle of incidence for a short period of time during a scan. The scanning beam will intercept the incident signal for only 1/Nth of the scanning period. The sampling loss in db is given by 10 log N. This degrades the sensitivity to that of a single element of the array or less. The present invention creates multiple scanning beams which are used to eliminate the sampling loss of the prior art.